1. Field of the Invention
The present invention relates to a high density optical disc reproducing apparatus, and more particularly, to an equalizer for a high density optical disc reproducing apparatus and an equalizing method therefor, capable of increasing a signal-to-noise (SNR) ratio and reducing inter-symbol interference (ISI) of a signal.
2. Description of the Related Art
Limit Equalizers (LE) can improve a signal-to-noise (SNR) ratio of output signals of linear equalizers used for high density optical disc reproducing apparatuses. An example of an LE is shown in FIG. 1, wherein an input terminal of the LE is connected to an output terminal of a linear equalizer (not shown).
In FIG. 1, an input signal and an output signal of the LE are respectively denoted by x(t) and y(t). The LE comprises a limiter 1; a four-tap filter consisting of elementary delay lines 2, 3, 4, and 5, tap coefficients 6, 7, 8, and 9, and a first adder 10; and a time compensation circuit consisting of elementary delay lines 12 and 13 and a second adder 11.
The limiter 1 of the LE cuts out a portion of the input signal x(t) above a reference value UT or below a reference value −UT, wherein the reference values UT and −UT are threshold levels of the limiter 1. The output signal y(t) of the LE is calculated according to Equation 1 below.
                                                                        y                ⁡                                  (                  t                  )                                            =                                                x                  ⁡                                      (                                          t                      -                                              2                        ⁢                        T                                                              )                                                  -                                                      ku                    1                                    ⁡                                      (                    t                    )                                                  +                                                      ku                    2                                    ⁡                                      (                    t                    )                                                  +                                                      ku                    4                                    ⁡                                      (                    t                    )                                                  -                                                      ku                    5                                    ⁡                                      (                    t                    )                                                                                                                          =                                                x                  ⁡                                      (                                          t                      -                                              2                        ⁢                        T                                                              )                                                  -                                                      ku                    1                                    ⁡                                      (                    t                    )                                                  +                                                      ku                    1                                    ⁡                                      (                                          t                      -                      T                                        )                                                  +                                                      ku                    1                                    ⁡                                      (                                          t                      -                                              3                        ⁢                        T                                                              )                                                  -                                                      ku                    1                                    ⁡                                      (                                          t                      -                                              4                        ⁢                        T                                                              )                                                                                                          (        1        )            
Here, T is a delay time of the elementary delay lines.
The LE operates differently for |x(t)|<uT and |x(t)|>uT.
In the case of |x(t)|<uT, the output signal of the limiter 1 is the same as the input signal thereof. Accordingly, the output signal y(t) of the LE can be rewritten as the following Equation 2.y(t)=−kx(t)+kx(t−T)+x(t−2T)+kx(t−3T)−kx(t−4T)  (2)
Equation 2 s the description of a Finite Impulse Response (FIR) filter that boosts the higher frequency spectrum components of the input signal x(t). Taking into consideration that the spectrum density of the pick-up noise (not shown) is large at lower frequencies, such boost is of short-length and increases a signal-to-noise ratio (SNR) for high-frequency components of the signal.
When |x(t)|>uT (that is true for large-length components of the signal), the boost of the filter completely diminishes. For example, when u1(t)=u2(t)=UT and u4(t)=u5(t)=−UT, the output signal of the first adder 10 is equal to zero, and the output signal of the LE is equal to the delayed input signal: y(t)=x(t−2T).
Any frequency boost is absent in this case. Therefore, the LE does not create its own Inter-Symbol Interference (ISI), however it does not reduce the ISI of the input signal. In other words, the ISI of the output signal of the LE is equal to the ISI of the input signal thereof. The large ISI of the output signal of the LE creates a substantial jitter and a substantial bit error rate (BER) in the high density optical disc reproducing apparatus. The reduction of the ISI is very desirable because a low ISI allows the use of optical disks with larger recording density.
However, the drawback of the LE lies in the high level of ISI of its output signal.
Related documents to this application include the article “New Equalizer to Improve Signal-to-Noise Ratio,” by Shogo Miyanabe, Hiroki Kuribayashi, and Kaoru Yamamoto, Jpn. J. Appl, Phys. Vol.38(1999) pp. 1715-1719, and U.S. Pat. No. 6,292,450, “Method of Automatically Controlling Bandwidth of Waveform Equalizer.”